In order to reduce atmospheric pollution caused by the emission of potentially harmful substances from engines, legislation has been introduced in the USA and the European Union to progressively lower legally binding limits for certain emissions. Similar limits have been adopted in other territories, but often lag some years behind the USA and EU For off-highway vehicles the limits have tended to have been introduced later than the equivalent on-highway engines. However, one of the biggest challenges for off-highway is that one engine will be fitted to a much wider variety of applications. This causes difficulties in ensuring that all engines meet the emissions limits regardless of application or installation configuration.
In the EU Stage IV of the limits enters into force in 2014, with higher output engines in the range of 130 to 560 kW needing compliance by January 2014, and lower output engines in the range of 35 to 130 kW needing compliance by October. The limits, testing regime and timetable are set out in EU Directive 97/68 and relevant daughter directives.
The corresponding US Legislation is referred to as Tier 4, and its introduction is staged between 2011 and 2014, with the “Tier 4 Final” standard needing to be fully met by the end of 2014. This legislation is set out in U.S Code of Federal Regulations (CFR). Title 40: Protection of the Environment Parts 1039, 1065 and 1068 plus all relevant guidance notes. A significant difference between the interim Tier 4 limit and Tier 4 final is a reduction in the oxides of nitrogen (NOx) limit.
Although the full requirements of this legislation are readily available, the key limits for NOx and PMs are 0.40 g/kWh and 0.02 g/kWh respectively for engines with power outputs in the range of 56 to 560 kW for US Tier 4 final, and 0.40 g/kWh and 0.025 g/kWh for EU Stage IV. The EU Stage IV legislation also limits ammonia emissions to a mean 25 ppm over the test cycle. Engines are to be tested in both steady state operation, and when following a US EPA and EU agreed nonroad transient cycle (NRTC). This cycle includes a cold start test which is weighted as 5% of the US test, and 10% of the EU tests.
It is usual for other countries or regions to adopt the same or similar emissions standards as the USA and EU, often with a later introduction date.
Various emissions abatement technologies are known for diesel engines, which may be used alone or in combinations to meet these limits. These technologies include the following:
Exhaust gas recirculation (EGR) systems which recirculate, under particular operating engine conditions, a portion of an engine's exhaust gas back into the engine combustion chambers (and typically cools that gas before it is introduced). This tends to lower combustion chamber temperatures. Since the production of oxides of nitrogen (NOx) increases at elevated combustion temperatures, lowering the temperature is an effective way of inhibiting inhibit the production of NOx. However, a potential downside of EGR is an increased production of particulate matter (PM) by the engine.
Diesel particulate filters (DPF) are provided to remove PMs from engine exhausts. As the particulate matter may accumulate in the filter causing blockages and mechanisms are required to clean the filter. Passive filters use a catalyst to remove accumulations, but need high temperatures to work. This cannot be guaranteed for off-highway applications, because the engine may spend a significant time idling, such that sufficient temperatures may not be achieved passively. An “active” DPF is therefore desirable for the technology to be effective in off-highway applications. Active DPFs periodically burn fuel, either in a fuel burner, or by using the engine management system to increase exhaust temperature by changing the fuel injection strategy to heat the filter to PM combustion temperatures. High reductions in PMs are achieveable (up to 99% in optimal conditions). Disadvantages of DPFs include an increased specific fuel consumption, and problems with the management of the extra heat expelled to other vehicle components or the surrounding environment, due to the higher exhaust temperatures. Consequently further heat mitigation measures may be needed. Additionally, the use of an a DPF upstream of other treatment units means that those other treatment units my need to use materials resistant to higher temperatures.
Diesel oxidation catalysts (DOC) utilise high surface area palladium and platinum catalysts to reduce hydrocarbons (HC) and carbon monoxide (CO) by a simple oxidation mechanism. As a result of the reduction in HC, there can also be a reduction in the mass of PMs, typically of the order of 20%.
Particulate oxidation catalysts (POC) use a contorted path to trap and remove some PM. The effectiveness of PM removal is typically between that of a DPF and DOC.
Selective catalytic reduction (SCR) combines the use of a catalyst such as vanadium, tungsten, copper zeolite (Cu-Zeolite), or iron zeolite (Fe-Zeolite) with a reductant such as anhydrous ammonia, aqueous ammonia, or more typically, urea, to convert NO and NO2 to nitrogen and water. Urea is typically used as the reductant, but has to be injected into the exhaust somewhat upstream of the SCR catalyst in order to thermally decompose into ammonia by the point at which it enters the SCR catalyst. Urea is preferred over ammonia, as it substantially safer to store and transport. In the USA, commercially available urea for use with SCRs is referred to as Diesel Exhaust Fluid (DEF), whereas in Europe it is referred to as “AdBlue®”. For SCRs to function effectively at the lower end of the temperature spectrum it has hitherto been desirable for there to be a 50:50 split of NO and NO2, although Cu-Zeolite catalysts have been found to improve performance at temperatures of less than 300° C. when there is little NO2 available. An advantage of SCR is that it has minimal impact of specific fuel consumption. Disadvantages include the need to additionally replenish the reductant on a periodic basis, and to provide space on a vehicle to package a reservoir of reductant. Typically, reductant usage is 1-7% that of diesel consumption. Further, there is a risk that excess injection of urea reductant, or that ammonia resident in the SCR catalyst at lower temperatures and released as the catalyst heats, causes unreacted ammonia to be emitted from the SCR into the atmosphere. This is referred to in the industry as “ammonia slip”.
Ammonia slip catalysts (ASC)—also known as or ammonia oxidation (AMOX) Catalysts—may be provided downstream of an SCR to oxidise ammonia to nitrogen and water and therefore prevent its escape into the atmosphere.
Lean NOx traps (LNT)—also known as NOx adsorber catalysts (NAC)—act to hold NOx until it the capacity of the adsorber is reached. At this point they may be regenerated by running the engine rich for a period of time. A so-called active LNT may be located upstream of a passive SCR. Ammonia that is generated during regeneration of the LNT may then be used in the SCR as a way of improving NOx removal. However, the periodic rich running required for regeneration is easier to achieve in smaller engines of light passenger vehicles up to a capacity of around 2.0-2.5 liters
The off-highway engine industry has a prevailing view that to meet both the PM and NOx limits of Tier 4 Final/Stage 4, aftertreatment will require a DOC, DPF, SCR and ASC in series, or at the very least a DOC, SCR and ASC. In particular, marketing literature from manufacturers may refer to “SCR only solutions”, whereas these solutions in fact additionally comprise a DOC, and/or comprise two SCRs. These aftertreatment solutions may add significant cost to an overall engine system, and/or may reduce the engine's fuel economy.
The present invention seeks to overcome, or at least mitigate the problems of the prior art.